The efficiency of a gas turbine generally increases with increased combustion gas temperatures. However, excessive temperatures within the turbine may reduce the longevity of the airfoils in the turbine and may thus increase repairs, maintenance, and outages associated therewith. As a result, various designs and methods have been developed to provide cooling to the airfoils. For example, a thermal barrier coating may be applied to an outer surface of the airfoil to enhance thermal protection. In particular designs, a cooling or purge medium may be supplied to a cavity or cooling circuit that is formed inside the airfoil to convectively and/or conductively remove heat from the airfoil. The purge medium generally flows in a flow direction which is substantially parallel to an inner surface of the cavity.
Cooling holes may be formed in the airfoil and may provide for fluid communication of the purge/cooling medium out of the cooling circuit and onto or across the outer surface of the airfoil, thus providing film cooling to the outer surface. The cooling holes generally extend through the outer surface and the inner surface of the cavity. In particular configurations, the cooling holes also extend through the thermal barrier coating.
The cooling holes are often drilled or machined into the high alloy metal of the airfoil at precise locations and in precise geometries after casting to optimize the cooling media flow over the airfoil. In particular instances, at least some of the cooling holes are drilled into the airfoil at various angles. For example, the cooling holes may be angled with respect to the outer surface of the airfoil, the inner surface of the cavity, the purge medium flow direction and/or with respect to a radial plane that extends perpendicularly to an axial plane of the airfoil.
Various processes are known for forming the cooling holes. For example, a liquid-jet guided laser may be used to create the cooling holes through the airfoil with a reduced risk of chipping the thermal barrier coating. The cooling hole forms in a generally conical fashion as the laser beam ablates the airfoil material. As a result, the laser beam will initially penetrate the inner wall of the cavity before the cooling hole is fully formed. A purge medium may be used to flush or purge the cavity of grit or other by-products which may form therein during the cutting process.
Because the cooling holes are generally angled or oriented such that a centerline of the cooling hole forms an obtuse angle with respect to the purge medium flow direction, the purge medium will flow or back-flow into the partially formed cooling hole around the laser beam, thus displacing the fluid column and preventing a properly formed cooling hole. As a result, the airfoil may require additional machining or may be scrapped, thus increasing time and/or costs to manufacture the airfoil. Therefore an improved system and method for manufacturing an airfoil, particularly for cutting or forming cooling holes within the airfoil would be useful.